Optical fibres made of silica or silica doped with germanium, boron or phosphorus in particular provide a very attractive means of long-distance transmission of telephone and television data because they have low attenuation for short-wave infrared radiation (800 to 1300 nanometers). Therefore, it would have been desirable to use them for undersea links if, up till now, no obstacles and cropped up. In particular, fibres are very sensitive to the combined effects of moisture and pressure which prevail at great depths, and to mechanical stresses which further weaken them by microcurves or even the pure and simple breakage of the fibres under a relatively low tractive force. Further, for long distances, it is necessary to place repeaters at intervals along the line. This requires the use of remote-supply electric conductors because the fibres are not electrically conductive.
It has effectively been proposed in the past to protect optical fibres in a metal casing, but protection thus conferred is not sufficient in itself to warrant the use of optical fibres for undersea transmission.
Preferred embodiments of the invention remedy these disadvantages and provide an optical fibre undersea cable which can be sunk to great depths, has low attentuation per unit length, is sufficiently strong to withstand various laying or raising handling operations and to which metal conductors can be added.